Process for the production of shaped parts of polyethylene



March 27, 1962 K. BECK ETA]. 3,026,570

PROCESS FOR THE PRODUCTION OF SHAPED PARTS OF POLYETHYLENE Filed June 10, 1957 lo Finishin Operation T'TTW 1 F a a i L J m bg :F f 1 (OH 21.2 3 (a a? L L l y t A? f g I J a f -['1 Wk E m \JAJ a g T3 lfiw/fizvk 4/01 VWfla E INVENTOR-T- a;

United States Patent 3,026,570 PROCESS FOR THE PRODUCTION OF SHAPED PARTS OF POLYETHYLENE Kurt Beck, Vreden, \Vestphalia, and ()tto Roelen, Oberhausen-Holten, Germany, assignors to Ruhrchemie Aktiengesellschaft, Oberhausen-Holten, Germany Filed June 10, 1957, Ser. No. 664,515 Claims priority, application Germany June 21, 1956 1 Claim. (Cl. 18-55) The invention relates to a process for the production of shaped parts of polyethylene which have a particularly high impact and shock resistance.

It has been found that shaped parts having particularly high impact and shock resistance, 'as well as accuracy and stability of measurements, can be produced of polyethylene with very good success if finely divided polyethylene, especially polyethylene powder having molecular weights above 500,000, e.g., above 1,000,000, is compressed in several pressure stages with the pressing in the second compression stage, and, if necessary or desired, in the subsequent compression stages being carried out under the simultaneous action of heat, until the material is plastified, whereupon the plastified material is further compressed at room temperature in a final compression stage.

Thus, stampings having larger dimensions than those of the finished parts are first produced by mechanical pressure, and preferably at room temperature. These stampings are subsequently converted into sintered and plastified polyethylene by the simultaneous use of heat and the renewed action of mechanical pressure.

It has been found advantageous, especially for loom fittings, to bring about the shape desired by cutting operations after the material has been plastified. By the cutting operation, the parts get approximately those dimensions intended for the finished parts, but these dimensions should still be chosen somewhat larger than those of the desired finished part. Finally, by applying once again mechanical pressure at room temperature, the shaped parts are brought into that state in which they finally are of accurate size, which they will maintain. If required, as in case of loom fittings, further cutting operations such as milling, drilling, punching, etc., may be subsequently carried out.

The compression in the last stage should be preferably carried out at higher pressure than in the sintering stage.

Particularly good results are obtained in the process of the invention when polyethylenes are used which have been obtained by polymerization of ethylene at pressures below about 100 kg./sq. cm. and temperatures up to about 100 C. with the use of catalysts consisting of mixtures of organometallic compounds, especially aluminum alkyl compounds, with compounds of metals of the 4th to 6th subgroups of the periodic system, especially titanium compounds, eg. titanium tetrachloride (see Angewandte Chemie," vol. 67, pages 541547 (1955)).

The invention is based on the discovery that it is required for simultaneously obtaining increased strength and accuracy of size that the total pressure to be applied for this purpose be subdivided into several stages. It is already known to produce shaped parts of polyethylene by cold pressing and subsequent sintering under pressure. It was observed, however, that the parts produced in this manner show a resilience similar to that of rubber, which is evidenced by the fact that the reduction in volume obtained in hot pressing is cancelled out when subjecting the parts to mechanical and/or thermal stresses. Therefore, neither increased mechanical strength nor dimensional stability can be obtained in this manner.

The process of the invention eliminates this disadvantage by applying at least a third mechanical compressing, thereby permanently removing the above-mentioned elastic recovery of the shape at room temperature or approximately room temperature. It was possible, therefore, to produce articles of polyethylene which were insensitive to most severe stresses by shock or impact to an extent not known up to the present for any other nonmetallic material.

The process of the invention is of the particular importance for the production of loom fittings. It is possible, for example, to produce pickers for looms, the life of which under identical service conditions is twice to three times or more that of the rawhide pickers previously used.

The total pressure to be applied may also be subdivided into more than three stages. For example, it may be favorable in sintering to reach the final pressure in several successive pressure stages. Moreover, in compressing the sintered part to dimensional stability, it may likewise be preferable to use several pressure stages.

It is essential in the operation of the process that the pressures applied in the hot treatment and in the subsequent cold pressing be in an appropriate ratio. The pressures applied at the end of the treatment should be highest and preferably be a multiple of those used in hot pressing. The pressures given in this description are the specific pressures in terms of kg./sq. cm. rather than the absolute pressures on the whole surface of any part.

The sintering step, for example, may be effected at pressures up to kg./sq. cm.; for reasons of keeping the size of the technical equipment small, it is desirable to keep the pressures within this low range. Moreover, it is not advantageous in connection with the desired properties of the material to apply much higher pressures to the heated material. On the other hand, it has been found to be advantageous to effect the third or subsequent pressing of the sintered material with substantially much higher pressures, as, for example, 200l000 kg./sq. cm. As experience has shown, parts obtained by multi-stage pressing in the manner described above retain, in addition to unreduced toughness, the desired shock and impact-resistance which is due to the high density.

The process permits the manufacture of shaped parts, subjected to shock and impact, for the construction of various apparauts, for electrical engineering, as well as for gears. Shaped parts, produced in accordance with the invention, are, moreover, particularly suitable for sound-absorbing shock spots which have to retain dimensional stability to a more or less high degree. The production of hammers and handles is also possible. However, the particular importance of the process resides in the manufacture of loom fittings such as take-up rollers, feed rolls, buffers, picker protections, pickers, spindle rings, and the like.

The invention will now be described in a specific example, but it should be understood that this is not given by way of limitation and that many changes can be made without departing from the spirit of the invention.

Example The polymerization of ethylene was effected in a vessel of glass with stirrer having a capacity of about 5 liters, into which were filled 2 liters of a C C hydrocarbon fraction from the hydrogenation of carbon monoxide. This hydrocarbon fraotion had been prepared by a hydrogenation effected at 250 C., subsequent refining with sulfuric acid, and intensive drying. After flushing of the reaction vessel with ethylene gas and heating to about 50 C., the catalyst solution was added while stirring and passing ethylene through the vessel. The catalyst solution had been prepared by mixing together 100 cc. of the same C S hydrocarbon fraction, 1.08 grams of diethyl aluminum monochloride, and 0.42 gram of titanium tetrachloride, and vigorously shaking the mixture for about 30 minutes. Upon addition of the catalyst solution, the reaction temperature was adjusted to about 75 C. After a reaction time of 12 hours, 472 grams of polyethylene had formed.

The mixture was filtered and the filter residue was treated with five times its quantity of 1% aqueous sodium hydroxide solution in a stirring flask. The mixture was then heated to the boiling point, thereby distilling off the residues of the hydrocarbon fraction to gcther with the water vapor.

The residue from distillation was finally washed with water to free it from alkali, and was then dried. The polyethylene obtained had a molecular weight of 1,100,- 000, as determined viscosimetrically.

This polyethylene powder, at a specific pressure of about 110 kg./sq. cm., was compressed into a sheet of 30 mm. thickness and 100 X 80 cm. size. This sheet was subsequently plastified for 8 hours between heated metal plates at a temperature of 150 C. and a specific pressure of 10 kg./sq. cm. Cooling was effected under the same pressure. There was obtained a sheet of plastified polyethylene having a density of 0.96. From this sheet, picker slugs were produced by cutting. The slugs were subsequently separately stamped at a pressure of about 750 kg./sq. cm., i.e. shaped by cold-pressing, whereby they were given their final shape. When used in a loom, the pickers produced in this manner resisted to 14 million picks, while rawhide pickers of the same shape were no longer usable after only 4.5 million picks.

The accompanying flowsheet shows a diagram or the operation according to the present invention. The polyethylene is fed from a storage hopper by way of a scale into the first press. From there, the pretreated material enters the heated press. The plastification and sintering steps both are carried out in this heated press. The material then is given its final forming in the last press and thereafter is removed to the finishing operation, not

shown.

The terms plastification and plastified as used in this specification denote that the polyethylene has been brought into a plastic state.

What we claim is:

A process for the production of shaped parts of polyethylene, said parts having particularly high shockand impact resistance, accuracy of size and size retention, which comprises compressing finely divided polyethylene, having a molecular weight above 500,000, in a first stage to approximately kg./cm. at room temperature to effect sintering; lowering the compression, in a second pressure stage, to approximately 10 kg./cm. at approximately 0., thereby effecting complete plastification of the polyethylene; cooling the polyethylene to room temperature while maintaining the same pressure; and compressing at room temperature to approximately 2001,000 kg./cm. in a final stage.

References Cited in the file of this patent UNITED STATES PATENTS 2,232,475 Renfrew et al. Feb. 18, 1941 2,736,925 Heisler et al Mar. 6, 1956 2,781,552 Gray Feb. 19, 1957 OTHER REFERENCES Article: Low-Pressure Polymers, Rubber and Plastics Age, vol. 36, November 1955, pp. 665-666. 

